It is known to purify waste waters using Fenton's Reagent (illustratively, European patent document B 0 022 525; German patent document C 38 32 523; H. Schwarzer, "gfw-Wasser/Abwasser" 129, #7, 1988, pp 484-19 1; E. Gilbert, "Nom Wasser" 62, 1984, pp 307-320), By means of chemical oxidation, the values of COD (chemical oxygen demand), BOD5 (biochemical oxygen demand after 5 days), TOC (total organic carbon), TOD (total oxygen demand), AOX (halogenated hydrocarbons adsorbable on activated carbon), but also organic compounds such as phenols, formaldehyde etc. and further toxic and inhibiting inorganic compounds such as cyanides, sulfur compounds, are reacted or eliminated. The Fenton reaction takes place in the acid range with addition of hydrogen peroxide as the oxidant and with iron(II) salts as the catalyst. In the ensuing neutralization, the formed iron(III) hydroxide precipitates in brown and hardly soluble form and is separated by sedimentation (gravitation) or by mechanical separation from the treated waste water.
It is further known that this resulting solid, which is called the Fenton sludge, adsorptively bonds organic contaminants to the sludge flakes. As a result difficult problems ensue concerning its treatment and elimination.
Accordingly it is the object of the present invention to create a process solving these consequential problems in simple manner, the Fenton sludges then being reprocessable, whereby simultaneously they may be re-used. This problem is solved by the object of the present invention.
The object of the present invention is a process to purify waste waters by chemical oxidation using Fenton's reagent in the form of hydrogen peroxide and iron(II) compounds in the acid range with ensuing precipitation of the iron(III) compounds in the weakly acid to alkaline range. This process is characterized in that the sludge containing iron (III), which is obtained besides the purified water, is acidified to a pH of less than about 4, to dissolve the iron (III) and to form an iron (III) solution, which then is reduced electrochemically back into iron(II) compounds, The reduced solution is fed back into the process.
Appropriate modes of implementation of this process are disclosed herein.
It is already known from the German patent document C 38 32 523 to separate the precipitated iron sludge and to suspend it in further waste waters to be treated; the purpose is to lower the operational costs of this method and to decrease the amount of iron sludge and the entailed special waste. However, this method does not allow satisfactorily solving of the consequential problems of treating and eliminating the Fenton sludge.